1. Field of the Invention
The present invention relates generally to apparatus for measuring and indicating incremental pressure fluctuations experienced within a closed system. More specifically, the present invention relates to an apparatus and method for measuring and indicating incremental pressure fluctuations within a container, tank, or pressure vessel.
2. State of the Art
Pressurized containers and/or tanks are used for a multitude of different applications and are typically filled, at least partially emptied, and refilled over and over again. As known in the art, the pressure within a closed container relates to the stress developed therein. So-called hoop stresses as well as axial stresses develop within a closed container due, in part, to the pressure therein. Since pressurized containers may be subjected to varying pressures (and, thus, varying stresses) by way of repeated emptying and filling, it may be advantageous to monitor, measure, and record incremental pressure fluctuations therein.
Moreover, stresses and, more particularly, cyclic stresses are known to cause failure in metals and other materials at stress levels considerably below their tensile strength. Typically, materials may be tested by applying a cyclic stress (via a rotating beam or other test protocol) and measuring the number of fluctuations that occurs before failure of the material. Materials may exhibit an endurance limit or endurance strength indicating a stress level below which the material will theoretically never fail.
However, many other considerations may influence the fatigue life of a closed container or tank. For instance, corrosion and initial surface finish of the materials used to fabricate the container may influence the ultimate strength and fatigue strength thereof. In addition, welds may be commonly used to fabricate tanks or containers and may be subject to particular fatigue and stress considerations. Thermal stresses and temperature influences may also affect the ultimate life of a pressurized container or tank as well.
Therefore, it may be useful to determine the number of times a particular magnitude of incremental pressure fluctuation is experienced within a closed container or tank by way of an incremental pressurization fluctuation counter. Such knowledge may be used to determine life expectancy, maintenance, repair, or inspection. Determining incremental pressure fluctuations may be advantageous over prior art approaches for indicating pressure fluctuations.
Containers may be filled and refilled at various times and under various conditions and therefore may experience various pressure changes or fluctuations. Such a container may be partially empty and at some initial pressure prior to filling. After filling, the container exhibits an increase in pressure. Of course, the overall change in pressure during filling may depend on environmental factors, such as temperature. Thus, the pressure change within the container depends on the initial pressure within the container as well as the pressure within the container after filling. Since the process of filling a container is normally not precisely controlled, the initial pressure as well as the pressure after filling may be variable. Therefore, it may be desired to measure relative or incremental pressure fluctuations during filling of a container in a simple, efficient manner.
Further, only relatively rapid pressure changes may be of interest, so that the slow emptying of the container may not be desired to be recorded or indicated. Alternatively, only pressure changes that occur during the filling of the container may be of interest.
Prior art approaches have described mechanisms to measure pressures, and even count pressure magnitudes within containers; however, the prior art does not disclose an incremental pressurization fluctuation counter. Thus, prior art approaches may record or otherwise indicate pressure cycles due to emptying of the container, or due to relatively slow changes in pressure, such as due to temperature changes.
U.S. Pat. No. 4,155,261 to Hesse et al. describes an arrangement for registering pressure fluctuations in a pressure system wherein a spring-biased piston may engage and move a free-wheeling clutch and thereby register an amount of pressure fluctuation according to the spring constants and the relationship between the rotation of the clutch and movement of the piston.
U.S. Pat. No. 5,517,008 (the '008 patent) to Francart, Jr. describes a mechanical fluctuation counter for counting fluctuations of a steam-powered liquid pump. More specifically, the '008 Patent describes a movable piston that may be biased toward a counter mechanism by pressurized steam and thereby engage the counter mechanism. Further, a compression spring may be used to bias the piston away from the counter in the absence of sufficient steam pressure. Each fluctuation being counted includes an increase, then a decrease, of pressure against the movable piston. The piston is extended on a piston rod against steam or gas pressure by a coil spring concentrically surrounding the piston rod. The free end of the piston rod drives a mechanical counter. No assist to the spring force is provided by sealing an initial pressure in communication with the piston.
In view of the foregoing, an incremental fluctuation counter which improves on conventional fluctuation counters and eliminates some of their respective disadvantages would be desirable.